Field of the Invention
The technical field of the invention relates to ion analysis using mass spectrometry. More particularly, to the development of a segmented linear ion trap to enable an extended range of ion processing techniques applied sequentially and facilitated by controlling the RF and DC electrical potential of trapping regions. Specifically, to the development of electronics and associated new techniques for ion trap operation.
Background Information
Linear ion traps have evolved into extremely powerful and versatile analytical devices and constitute a significant and indispensible instrumentation section in modern mass spectrometry. Deployed as stand-alone mass analyzers or integrated in hybrid mass spectrometers, the range of tools and methods available for manipulating gas phase ions are remarkably wide. Linear ion traps are ideal platforms for developing and testing novel designs to achieve enhanced performance capabilities and further extend versatility. Reviews on linear ion trap instrumentation are concerned with 2-dimensional RF trapping fields and the properties of radial ion confinement, axial control of ion motion including approaches for coupling to mass analyzers [Douglas et al, Mass Spectrom Rev 24, 1, 2005].
The two main advantages of linear ion traps compared to the standard 3D quadrupole ion trap include reduced space charge effects due to the increased ion storage volume and enhanced sensitivity for externally injected ions due to higher trapping efficiencies [Schwartz et al, J Am Soc Mass Spectrom 13, 659, 2002]. Enhanced performance has been demonstrated in a dual-pressure linear ion trap where ion selection and fragmentation process are optimized independently [Second et al, Anal Chem 81, 7757 2009]. More complex arrangements involve mass selective axial ejection techniques either based on fringe fields to convert radial ion excitation to axial motion [Londry & Hager, J Am Soc Mass Spectrom 14, 1130, 2003] or on the use of vane lenses inserted between RF pole-electrodes and supplied with axial AC excitation waveforms [Hashimoto et al, J Am Soc Mass Spectrom 17, 685, 2006]. The activation-dissociation methods available are limited to Collision Induced Dissociation (CID) and Electron Transfer Dissociation (ETD) and so far no more than two activation methods can be performed in tandem in the same linear ion trap. Therefore, the development of novel designs capable of supporting a wide range of efficient activation-dissociation tools and methods and the ability to perform these sequentially is essential, particularly for the analysis of highly complex biological samples and proteins.
A concept design of a collision cell with multiple potential regions for storing and processing ions is disclosed in U.S. Pat. No. 7,312,442B2. Although the proposed ability to sequentially activate and dissociate ions using different techniques is highly desirable, the method neither involves injection of charged particle beams for dissociative interactions nor is concerned with adjusting the DC electrical potential and consequently the potential energy of the ions between multiple levels, which greatly facilitates control of the interaction energy to optimize activation-dissociation processes. Furthermore, advanced control of the DC electrical potential and also the ion potential energy are critical for efficient ion transfer between trapping regions including receiving and releasing ions with precise kinetic energy to and from a linear ion trap respectively. These new aspects require novel DC switching technology and methods disclosed in the present invention.
Techniques to control the interaction energy between ions stored in ion traps and externally injected electrons are disclosed in U.S. Pat. No. 7,755,034B2. In order to control the energy of the interaction in a linear ion trap three-state digital waveforms are employed where electrons are injected during the intermediate voltage state. In addition to the constrains in the mass range of the ions stored successfully in the ion trap imposed by three-state RF trapping, the voltage amplitude accessible for the intermediate state is also severely limited and so is the accessible energy range available during interaction. Another disadvantage of the method disclosed for operating a linear ion trap is the narrow time window for interactions to occur, which is limited to less than ⅓ of the waveform period. The method disclosed in the present invention alleviates all these problems by decoupling the properties of the RF trapping waveform from the electron source potential by the superposition of DC field components to control the potential energy of the ions independently and to any desired level. The mass range remains unaffected over an unlimited energy range and the time of interaction is maximized.
Overall, the need remains for an improved linear ion trap and methods for activating and dissociating ions sequentially in a single apparatus using different techniques and performed with high efficiency.